Power converter system



Oct. 4, 1949. c. w. HANSELL 2,433,756

H POWER CONVERTER SYSTEM Filed Dec. 29. 1942 TO LOAD K E Y E D S H 0 RT PUL S E 70 GENERATOR May BE flm u TUDE Mam/Lamp I I\ I F I L HE AT 1 N6 SUPPLY 6 ,1 Jylvcmeo/vo US Cour/201.

lVH/CH May BE PHASE 0a FEEQUENC) MODULAJTED INVENTOR CLARENCE W. HANSELL ATTORN EY poses.

Patented Oct. 4, 1949 POWER CONVERTER SYSTEM Clarence W. Hansell, Port Jeiferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 29, 1942, Serial No. 470,437

16 Claims. 1

This invention relates to improvements in direct current to high frequency power converter systems, and has for its primary object to provide a method of and apparatus for increasing the efficiency of such converters.

Although the invention is hereinafter described with particular reference to vacuum tube amplifiers and oscillators generally; it should be clearly understood that the principles of the invention have wider application and are not limited to vacuum tubes. 7

In accordance with one specific embodiment of the invention, the efficiency of operation of vacuum tube amplifiers is greatly increased by causing the amplifier vacuum tubes to conduct or pass anode current in short but very high pulses at a relatively veryhigh anode potential, and to deliver pulses of high frequency current to an output circuit which will integrate the pulses into a continuous wave current and which possesses sufiicient frequency selectivity to remove the modulations of pulse frequency or higher. The output circuit is designed to pass the signal modulations for communication pur- The relatively high anode potentials are impressed upon the vacuum tube amplifiers for short periods of time and are higher than the rated voltages of the tubes for continuous operation. The signal modulations may be of an amplitude, phase or frequency character and should be of a difierent and considerably lower frequency than the pulse frequency. The principles underlying the design of suitable frequency selective output circuits for accomplishing the desired result are well known in the art, and such circuits may take the form of tuned circuits, filters or networks. Any suitable load circuit, such as a transmission line extending to an antenna, can be coupled to the frequency selective output circuit. It is preferred that the output circuitand the transmission line coupled thereto should be of the low power factor (i. e., low loss) type.

In order that the present invention may be better understood, a theoretical explanation will now be given. It is to be understood, however, 1

that thistheoretical explanation is given merely for the purpose of exposition, and while this explanation is believed to be correct, it is not of necessity complete, nor does the operation of the invention depend upon its accuracy or otherwise.

In making tests on RCA-955 type vacuum tubes having a normal maximum anode current rating of 100 milliamperes, it was found that tubes of this type were capable of passing emission currents ranging up over one ampere if these currents were caused to fiow in sufliciently short pulses. In making these measurements, the potential applied to the tube was placed on one axis of an oscilloscope and the current flowing to the tube was placed on the other axis. As aresult, the tube gave a trace of anode potential versus anode current during the period of the pulses; By watching these traces and varying the conditions of the experiment, it appeared that the very high peak currents were coming not from electron emission directly but from stored space charge in front of the cathode. The reason for this conclusion was that the peak currents which could be passed on very short pulses were considerably greater than the limiting or final current which could be passed for somewhat longer pulses. In other words, the electron current acted as if it were coming from a storage reservoir which could be emptied by taking sufficiently high current for a long enough time. These experiments indicate that vacuum tubes operated with pulses sufficiently short to employ only the electrons stored in the space charge are capable of providing enormously high instantaneous peak power outputs, and these peak power outputs should be obtainable with much greater power efiiciency and at much higher frequencies than are obtainable from tubes operated in the ordinary way. Thus, by operating high frequency amplifiers with short carrier wave pulses and integrating these pulses in a circuit too selective to respond to the pulses, we have a means of increasing the power efiiciency and frequency limit of vacuum tubes generally.

One alternative theory to account for the observed large amplitude of sufiiciently short pulses is that cathodes in vacuum tubes have composite surfaces such that areas with large thermionic emission are partially insulated from the underlying metal and so act as if they had condensers in series between them and the base metal. The high peak currents flow momentarily through the dielectric capacity but, as the capacity is discharged, large resistances in parallel with the capacities must carry the current and these resistances efiectively reduce currents from the areas giving the appearance of a reduction of emission.

Another theory is that ions of cathode materials, or of gas, are present in the space between anode and cathode and lower the potential drop through the vacuum tube but these ions are rapidly removed after application of anode-tocathode potential and therefore cause a rise in potential drop between anode and cathode.

Quite apart from these theories to account for the observed fact that vacuum tubes can pass very short peaks of current, of values much above the limiting steady state currents, it is well known that the output and efiiciency of vacuum tube amplifiers increase if the output load impedance and anode-to-cathode power potential are increased together.

A more detailed description of one embodiment of the invention follows in conjunction with the drawing, wherein the single figure illustrates the principles of the invention as applied to a radio communication system employing a vacuum tube amplifier circuit.

Referring to the drawing in, more detail, there is shown a pair of vacuum tubes I and 2 coupled together in a conventional push-pull amplifier cross-neutralization circuit. It will be noted that the grid of one tube is coupled to the anode of the other tube through a neutralizing condenser which serves the purpose of balancing or neutralizing the undesired grid to anode inter-electrode capacities. The grids of tubes I and 2 are connected to opposite sides of a tuned input circuit 3, while the anodes of these tubes are connected to opposite terminals of a tuned output circuit 4. This tuned output circuit is inductively coupled to a two-conductor transmission line 5 extending to a suitable load, such as an antenna. The tuned input circuit 3 has impressed upon it suitable high frequency continuous wave currents from a continuous wave high frequency source indicated conventionally by box 6. The apparatus in box 6 may include such equipment as a crystal oscillator followed by an amplifier frequency multiplier. The anodes of the tubes I and 2 are supplied with polarizing potential by means of a connection I, one end of which is connected to the mid-point of the inductance of the tuned output circuit 4 and the other end of which is connected to one winding of a pulse transformer 8. The other winding of the pulse transformer 8 is connected between a positive terminal of a source of unidirectional potential and the anode of a keyer vacuum tube 9. A short pulse generator I0, which is keyed by the signals to be transmitted, is connected between the grid and the cathode of the keyer tube 9.

By means of the keyer circuit, I am able to supply to the anodes of the vacuum tube amplifiers I and 2 a very high anode potential in pulses. This potential exceeds the rated voltages which can be applied to the anodes of the amplifier tubes for continuous operation. Putting it in other words, due to the heating considerations, there are voltage limitations for tubes which are necessary to prevent the tube from being destroyed. Because of the fact that I apply voltages to the anodes of the tubes in pulses, I am able to impress potentials on the electrodes of the tubes which are very much higher than the rated voltages of the vacuum tube. As an illustration, if the tubes I and 2 ordinarily may stand only 10,000 volts on the anodes and one ampere per tube of direct current input to the anodes, for continuous operation, I am able by means of the pulser system of the invention to momentarily apply to the anode 50,000 volts for, let us say, one-tenth of the time, and to obtain two amperes of direct current input to the anodes at this much higher voltage. Under the first condition of operation; namely, the steady state condition or continuous operation condition, it is reasonable to obtain an efliciency of 50% to, 60%, whereas under the second condition of operation (namely,

4 by supplying higher voltages in pulses to the anodes) I may get an efliciency of approximately 90%. In this example, the average radio frequency output in the first case may be 5 kilowatts, while in the second case it may be of 9 kilowatts.

As an illustration of the frequencies that might be employed, the continuous wave high frequency source 6 which drives the push-pull amplifier can supply current of a frequency of twenty megacycles to the tuned input circuit 3. The short pulsegenerator 9 can be of a type which produces 500,000 pulses per second, each pulse of which may be two-tenths of a microsecond long. In this way I am able to supply polarizing potentials to the anodes of the tubes I and 2 at a value which exceeds the steady state value and for a length of time which occupies only 10% of the total or steady state time. It is contemplated that the short pulse generator I0 should be keyed by the signals to be transmitted. Thus, if telegraph keying is employed, when the key is in the down position, the pulser system will key the vacuum tube amplifiers I and 2 at a rate of 500,000 pulses per second of twotenths of a microsecond duration for each pulse, whereas when the key is in the up position, there will be no anode potential supplied to the pushpull amplifier and, hence, there will be no output current in the tuned circuit 4. The output circuit 4 will thus receive pulses at the pulse rate, each pulse of which will contain high frequency energy of twenty megacycles per second, which is the frequency of the high frequency source 6. The output circuit 4 is merely illustrative of any suitable frequency selective circuit which is tuned to twenty megacycles and designed to respond and pass the signal modulations of the 20 megacycle current (keying or otherwise) of relatively low frequency, but which will not respond to the 500,000 cycle pulse frequency modulations of the twenty megacycles current. Where keying is employed, the keying modulation will usually be in a range below 500 dots or dashes per second. From what has been said before, it will be appreciated that the output circuit'4, which is too selective to respond to the 500,000 cycle pulse modulation, may be a tuned circuit or a filter.

In practice, the frequency selective output circuit 4 should be of the low power factor type (low loss). The transmission line 5 in this case should not be coupled too tightly to the output circuit; that is, the coupling between the line 5 and the output circuit 4 should not be as close as that customarily found in conventional continuous wave circuits. As a specific illustration, the output circuit 4 may be a resonant line circuit.

The pulse rate (that is, the pulse frequency of the generator I0) and the length of each pulse can vary so long as the pulse length does not become so short that it is impossible to produce the pulses. Putting it in other words, there are limitations on the shortness of the length of the pulses due to present equipment limitations and the fact that the pulse rate cannot be so low that the pulse modulations cannot be filtered out by the frequency selective outputcircuit 4.

Although the pulse keyer has been described in the specific embodiment illustrated in the drawing as being keyed for telegraphic operation,

. equipment for transmitting the signals. Current from the source 6 may be phase or frequency modulated when these types of modulation are desired for the currents in circuit 4 and transmission line 5. The direct current input potential to pulser 9 may be modulated when amplitude modulation of the current in circuit 4 is desired.

It should also be understood that output from source 6 may be pulsed more or less completely, synchronously with the pulsing of anode-tocathode potentials on tubes I and 2.

What is claimed is:

1. In combination, a converter of direct current to high frequency power, means coupled to said converter for keying the same including means for applying input power thereto in pulses of duration short compared to the time interval between pulses, a load circuit, and a frequency selective circuit between said converter and said load circuit, said frequency selective circuit being selective to pass frequencies other than said keying or pulse frequency but not to pass said keying or pulse frequency.

2. In combination, a vacuum tube system for converting direct current to high frequency power, a circuit for applying polarizing potentials to certain electrodes of said tube system in pulses which are short compared tothe time interval between them, said tube system including a frequency selective output circuit, and a load coupled to said output circuit, said output circuit being designed to integrate the pulses into a continuous wave current.

3. The combination with a vacuum tube whose input and output currents are 1imited by heating considerations and as a result thereof the conversion efficiency is low for continuous or steady state operation, a source of continuous wave high frequency current for said tube, of means for operating the tube in pulses repeated at a rate higher than the highest signal modulation frequency and at a higher voltage than could be applied continuously, a utilization circuit coupled to said tube, and a selective circuit located between said utilization circuit and said tube for removing the pulse modulations but not the signal modulations.

4. In a high frequency system, an electron discharge device having a cathode, an anode, and a control electrode, a circuit for applying continuous high frequency waves between said control electrode and said cathode, means for applying across said anode and cathode a difference of potential of such value as would destroy the device if it were to be applied continuously, said means including a circuit for periodically interrupting the electron discharge in said device at a rate considerably lower than the frequency of said continuous waves, and a frequency selective output circuit for said device designed to respond to the frequency of said continuous waves and to'remove modulations of the interruption frequency.

5. In a high frequency communication system, an electron discharge device having a cathode, an anode and a control electrode, a circuit for applying continuous high frequency waves between the control electrode and cathode, a pulser system for periodically impressing between said anode and cathode a difference of potential of such value as would destroy the device if it were to be applied continuously, a signal modulation circuit for said system, and a frequency selective output circuit for said device, said output circuit being tuned to the frequency of said continuous waves and designed to remove the pulse modulation frequency but not the signal modulations.

6. In a high frequency communication system, a push-pull amplifier having a pair of vacuum tubes each having a cathode, an anode and a control electrode, said amplifier having a tuned input circuit between the control electrodes and a tuned output circuit between the anodes of said tubes, a source of high constant frequency waves in the range of megacycles coupled to said input circuit, means including a pulse generator and a keyer electron discharge device for periodically applying a polarizing potential to said anodes of a value which would destroy the vacuum tubes if it were to be applied continuously, a signal keying circuit for keying said pulse generator at an audio frequency rate, said output circuit being tuned to the frequency of said source and designed to remove the pulse modulations but not the audio frequency signal modulations.

7. A system in accordance with claim 5, including a utilization circuit substantially loosely coupled to said output circuit, and characterized in this that said output circuit is a low power factor circuit.

8. In a high frequency communication system, an electron discharge device having a cathode, an anode and a control electrode, a circuit for applying continuous high frequency waves of the order of megacycles between the control electrode and cathode, a pulser system for periodically impressing between said anode and cathode a difference of potential of such value as would destroy the device if it were to be applied continuously, the total time interval during which said 5, difference of potential is impressed between said anode and cathode by said pulser system not exceeding substantially 10% of the steady state time, a signal modulation circuit having a frequency of operation appreciably lower than the interruption rate of said pulser system, and a frequency selective output circuit for said device, said output circuit being tuned to the frequency of said continuous waves and designed to remove the pulse modulation frequency but not the signal modulations.

9. In a high frequency communication system, a push-pull amplifier having a pair of vacuum tubes each having a cathode, an anode and a control electrode, said amplifier having a tuned input circuit between the control electrodes and a tuned output circuit between the anodes of said tubes, a source of high constant frequency waves in the range of megacycles coupled to said input circuit, means including a pulse generator and a keyer electron discharge device for periodically applying a polarizing potential to said anodes of a value which would destroy the vacuum tubes if it were to be applied continuously, said pulse generator producing pulses at a rate above audibility, a signal keying circuit for keying said pulse generator at an audio frequency rate, said output circuit being tuned to the frequency of said source and designed to remove the pulse modulations but not the audio frequency signal modulations.

10. In a high frequency system, an electron discharge device having a cathode, an anode, and a control electrode, a circuit for applying continuous high frequency waves between said control electrode and said cathode, means for applying across said anode and cathode a difference of potential of such value as would destroy the device if it were to be applied continuously, said means including a pulsing circuit for periodically interrupting the electron discharge in said device at a rate considerably lowerthan the frequency of said continuous waves, a signal modulation circuit for applying modulation. at a frequency appreciably different from the pulse interruption frequency, and a low-loss. frequency selective output circuit for said device designed to pass the signal modulationsof. saidcontinuous waves but not to respond. to said pulse interrups tion. frequency,. said selective output circuit serving. to integrate the pulses-into a; continuous wave-current.

l l'. In. a high frequency system, an electron discharge device having a cathode,an anode; and a; control electrode, a circuit for applying corrtinuous high frequency waves between saidv con:- tro'li electrode and said cathode, means for modulating said continuous high: frequency Waves, means for applying across said anode and oathode a difference of potentialof such. value as would destroy the device if it were to? be applied 2 cuit including a generator of waves of a frequency appreciably lower thanthe frequencyofisaid con.- tinuou Waves for operating the. tube in: pulses which areshort compared to the time interval between. them and repeated at a. rate'higher than the; highest signal modulation frequency and at ahigher voltage than could be applied continu.- ously, a low-loss selective output: circuit for said tube; said selective output circuit being selective toipass; the signa-l; modulations butnot the pulse modulations, and a frequency utilization. circuit coupled to said selective output circuit.

1-3; A pulse communication. system. comprising an; electron discharge device, a. source of continuous Wave carrier current: coupled to the input electrodes. of said. electron discharge device, means for modulating the phase or frequency of said: carrier current, a. pulsing device coupled: to the output electrode of said electron. discharge device: for interruptedly supplying said output electrode with an operating potential. ofsuch valueas would destroy the discharge deviceif it were applied continuously" and. at a rate appreciably lower than the frequency of said. carrier current; and means for applying signal modulations to said pulsing device.

M. A pulse communication system. comprising anelectron discharge device, a source of continuous wave carrier current. coupled to the input electrodes of said electron. discharge device,. means for modulating the; phase or frequency of said carrier current, a pulsing. device for interruptlng the operation. of said device at a fre quency different from the frequency of said carrier current, means for applying signal modulations to said. pulsing, device, and a synchronous control coupled toasaid source and to said pulsing device for controlling; the: exact timing. of the pulsing device relative to the modulation ofthe carrier current.

15. A pulse communication. system comprising an. electron discharge device producing rad-iofrequency energy inits output, a pulsing device coupled to the output. electrode of said discharge device for interruptedly supplying said output electrode with an operatingpotential of such value as would destroy the discharge device if, it were applied continuously and at a rate apprecis ably lower than. the frequency of said radio f-re quency energy, means for modulating a characteristic of the pulses. produced: by said pulsing device. means for modulating the phase or frequency of said-radio frequency energyand means for synchronously controlling the timing of. the pulsing device: and. said last modulating. means.

16; In combination, almultieelectrode electronic device system. for converting direct current to high frequency power,v a circuit: forv applying polarizing potentials to. certain electrodes of said device in pulses which are short. compared to. the time intervals. between them,- said system includa frequency. selective output. circuit, and a load coupled. to said. output circuit. said. output circuit beingdesigned. to integrate the pulses into a continuous wave currents CLARENCE. HANSELL.

REFERENCES CITED The following:- references are of record in the file; of this. patent:

UNITED STATES PA'IENTS.

Number Name Date 1,695,042 Fearing Dec. 1l-,,19-28 l',908;249 Hundr c i May 9; 1933 2,031,799 Koch c i Apr; 21, L936 2;1 (l3;090 Plebanski- Dec. 2t, 1-93-7 2 103 362 Hausa-1t l Dec: 28, 193? 2-.21,'4&9- Hofer ct: alt. (Dctt 29-, 1940 aces/tor Reeves c Dec. 16 .1941 2;338;51-2 Harmon a-. Jan. 4, I944 FGRLEIGN PATENTS Number Gountry- Date 544 665 Great Britain. .H Dec. 5, 1941 G IHEERI REFERFENGES Radio February, 1939,, "Pulsihg. Amateur Transmitters by. Albert. W... Friend, pages 28-34, and 82. 

